Hi all, newbie to the forum, pretty good (I think) amateur mechanic and trouble shooter.

So I bought a fixer-upper "KPX" atv/quad on Craigslist. They said they had it running, but I doubt it because for one the starter chain was broken and it was all torn apart. No nameplate on it. Fortunately I didn't pay much. :-)

It's got a 2 cylinder ~250cc engine on it, and the only identifying features I can find at present are the "KPX" embossing on the side covers.

After a bunch of fooling around, I got the starter chain replaced, cleaned all the accessible stator/magneto parts, and sorted out the wiring. In the process I zorched the CDI, which is a seemingly common 2 plug 6 pin. However, I replaced the carb, coils, and got a new CDI (China, ~$5, ebay, but it did say it could handle a 250). I've also am aware of the difference between an "AC" CDI and the "DC" version which requires a source. I don't know what this one is, but I've got to coils paralleled off the single CDI, and they both spark. It's a dual cab.

Anyway, I finally got it to cough and kind of idle, and what I'm facing now is that only one cylinder is running, and it only does it at 3/4 choke.

I've tried:
-swapping coils
-swapping plugs
-pulled the plug on the dead cylinder, case grounded it, and observed decent spark
-checked compression on the dead cylinder (90, inches I believe, cold, seems in spec)
-put my palm over the carb intake for the dead cylinder, and it sucked plenty of fuel through the jets
-cleaned the old jets from the old carb and put them in the new one
-took a look at the valve train, which looks find (though the big black widow is now living in a mcdonalds clear plastic ice cream cup). Valves all look like they are moving normally when I crank it.
-oil level is fine
-pulled off the exhaust pipes to make sure that a clogged exhaust wasn't it
-hit the dead cylinder with a little starting fluid and it fires then.

When I get it dialed in just right on the choke and throttle idle adjustment, I can get it to barely idle on the one cylinder, but if I hit the throttle it gags and dies, regardless of the above.

WHY is one cylinder not firing? Why isn't the other one running normally? What am I missing? Baffled. Help!

The only thing I can think of is the magneto for some reason is off it's normal position. Possible?

I'm going to a quick check to make sure it's in fact an AC CDI, but I have another just in case it's unhappy with 12V on that input. I'll keep posting any test results.

A couple other quick questions: could the CDI just not be compatible? They do have a timing curve in there, no? I can't imagine it's "backwards" or something and retarding vs advancing?

Any suggestions welcomed.

Thanks!

 

Update: I pulled the stator and valve covers and fished around. There appear to be two timing marks 180 deg apart on the cam shaft timing gear, which of course rotates 180 for every 360 on the crank shaft.

When one of the timing mark is lined up with the forward edge of the head: there are marks on the "flywheel", "T", "F", and a coupe of lines. "T" is pretty much at the top. "F" is advanced past tdc, and the other marks even beyond that. I guess "F" must mean "fire". Also, the left piston is pretty much at tdc )didn't check the right one but it should be the same), and the magnet lines up with the pickup. I'm guessing that "F" actually is when it is suppose to trigger the pickup, maybe 5-10 deg before TDC from the looks of it, and the CDI supposedly as an advance curve on that based on the RPM. The magnet is pretty wide, a few degrees in any case, maybe 5.

So without knowing more about this engine, that all looks right. It's coincidental I suppose, but maybe not...

I can post pic's if it helps diagnose.

 

Fairly sure you have a Honda Nighthawk/Rebel copy engine, Quadzilla made their name circa 2005 selling the 250 version, but it is really difficult to find info on setting carbs etc. If you shine a timing strobe light on the flywheel you should get the F mark lined up at a certain RPM. Pin out for CDI is non standard on Quadzillas and it has an AC CDI, could be different on other makes, but an extra coil on stator, with, I think, a black/red wire from it, would indicate AC.

 

Thanks merryman.

I think you are right on the clone. It looks like this one:

https://s.ecrater.com/stores/392826/...61_392826b.jpg

It does indeed have an CDI AC power coil, in fact it has two (and one less stator pole). They lead to a red/black wire, which it appears is the convention for that.

I don't actually know if the replacement CDI I have is AC or DC. Obviously it is firing, though maybe not reliably. Looking at it with the pins at the top, the DC version is supposed to have two ground pins at the bottom of the 4-pin connector, the AC input at the bottom of the 2-pin, and of course no kill input (ordinarily at the top of the 2-pin on the AC CDI).

This thing does have the 2 bottom pins of the 4-pin shorted (green--GND), but the kill also is connected and works (!). The original CDI, which I zorched accidentally, had 2 separate outputs--one for each coil--on the 4-pin connector. I modified my harness so that the coil primaries are spliced together. (They go to one of the top two of the 4-pin, I forget which, the other being the trigger pulse from the magneto).

So maybe it is a DC, though it has the kill function still built in for some reason. Anyway, I'm going to hook 12V to the power input pin and see if that makes any difference, but I have a performance AC unit on order.

As I said, one cylinder is not firing at all (cold exhaust), even though it is sucking fuel, has 90in compression, opened up exhaust, and I testing both plugs and coils on the other cylinder indicate they all work. So I can't really check the timing since it won't run consistently. I'll see if I can stick an inductive timing light to the plug wire on the bad side just to confirm it's firing the plug at least. However, it is mystifying to me as to why one cylinder would fire consistently and the other not at all if it's the CDI being somewhat intermittent.

There's one other thing with the jets I need to to look at too.

 

To close this out--

I replaced:
-CDI with high performance AC unit
-Plugs
-Coils
-Carb (before)

and then it ran.

Thanks all who responded.